In sensor technology, sensor arrays are increasingly being used in order to be able to detect a plurality of chemical compounds simultaneously. These sensor arrays are used especially in bioanalysis technology, e.g. for DNA analysis, for detecting pathogens or for examining metabolic disturbances. In this case, body fluids such as e.g. blood or urine are worked up by way of chemical reactions. They are then conducted via the sensor arrays and interactions between the sensors of the sensor arrays and the substances to be detected from the body fluids are examined by means of optical, frequency or electrochemical measurements. In this case, the sensors of the sensor arrays are often coated with molecules which react specifically with the substances to be detected from the body fluids. By means of washing with liquids, unbound substances can be removed from the surfaces of the sensors. The specifically bound substances to be detected can then be detected directly or indirectly, e.g. by means of markers.
Typical detection methods comprise optical measurements, frequency measurements or electrochemical measurements. Thus, by way of ellipsometry, for example, it is possible to examine the changes in the refraction properties of the surface after the binding of the substances to be detected. An alternative is the binding of specific, optically active markers to the substances to be detected. These can be detected e.g. by way of their fluorescence or visible color upon irradiation with specific wavelengths. Frequency measurements using a quartz microbalance can detect changes in the mass of molecules bound to the surface. Current-voltage measurements with the aid of electrodes in a liquid can electrochemically detect the binding of the substances to be detected, or the bound state thereof directly or by way of reaction products.
What the methods described have in common is that indefinite flow conditions above the sensors can lead to measurement errors. Depending on the flows, different double layers form at the boundary layers between the sensor surfaces and the liquid or molecules bind non-specifically to the surfaces, as a result of which e.g. the reactivity, the binding affinity and the optical, electrical and friction properties of the sensor surfaces change.
The production of constant flows above the sensors of the sensor arrays is complex and requires a high outlay in respect of apparatus. Therefore, measurement is preferably carried out with stationary liquids, that is to say without disturbing flows above the sensors.
The document DE 100 58 394 C1 describes an arrangement and a method for producing stationary liquids above sensors of sensor arrays. The sensors of a sensor array are arranged on a planar surface with walls between the individual sensors, wherein the walls project from the surface. A housing upper part is arranged parallel to the planar surface of the sensor array at a distance from the walls, said housing upper part being at a sufficient distance from the surface and the walls to allow liquid to flow between the surface and the housing upper part. Before the measurement, the housing upper part is pressed in the direction of the surface with walls with the aid of a plunger, such that the walls and the housing upper part are in close contact.
The flowing of the liquid has been prevented, and closed-off reaction spaces, delimited by the planar surface, the walls and the housing upper part, have been produced above the sensors. The liquid which has been enclosed in the reaction spaces can then be examined without exchange of liquid between different reaction spaces. The measurement of the sensors takes place with the aid of the closed-off reaction spaces independently of one another.
The method described in accordance with DE 100 58 394 C1 leads to an arrangement with housing upper part and plunger in which the walls between the sensors have to correspond to precise dimensions. These precise dimensions are difficult to realize in practice:
If the walls project too little from the planar surface in which the sensors are arranged, then this gives rise to reaction spaces which are too small and have an amount of liquid that is too small to carry out a meaningful measurement. The housing upper part can, under certain circumstances, be seated on the sensors at high plunger pressure, whereby the measurement is corrupted.
If the dimensions of the walls are made too large, then the sensors have to be arranged at a larger distance from one another, and the required amount of liquid for filling the arrangement increases. This means that the sensitivity of the measurement turns out to be lower and the number of sensors available for measurement decreases for the same area since the dimensions of said sensors have to be made larger in order to achieve a sufficient measurement accuracy.